In the manufacture of certain electrical cables, in particular medium or high tension cables, it is desirable to cover the cured insulating layer with a semiconducting layer of cured thermoplastic material that is easily detachable from the insulating layer. This requirement means that the adhesion between the two layers should be low. However, the adhesion should not be so low as to lead to the two layers coming unstuck when the cable is subjected to various stresses in use.
This particular characteristic is referred to by the term "strippability". It is generally defined by a value or a range of values which, under defined operating conditions, cause the two layers to separate.
For example, in France, Standard HN 33-S-23 published by ELECTRICITE DE FRANCE in November 1981 provides for a stripping test applied to a strip of semiconducting layer which is 10 mm wide and cut parallel to its generator lines. After one end of this strip has been lifted over a portion of its length and folded right back over itself, the remainder of the strip should become detached from the insulating layer when the axially-applied traction force lies in the range 5 newtons (N) to 25N, at a temperature lying in the range of 0.degree. C. to 40.degree. C. Similar tests exist in other countries.
A property which is defined so precisely cannot be obtained merely by using any semiconducting mixture, such as that which would result, for example, from incorporating conductive carbon black in a curable mixture of the underlying insulation.
In addition, it is clearing desirable for the assembly constituted by the insulating layer and the semiconducting layer to be capable of being cured in a single operation, which implies that both of them should require the same curing process. Thus, insulating and semiconducting mixtures are known which are capable of being cured chemically by means of peroxides. However this method requires equipment which is bulky and expensive. Further, it is not technically very satisfactory since it can facilitate the creation of water arborescence in the insulating layer which is highly prejudicial to the lifetime of the cable.